90%? Nope. Most fights never get past the first punch.
Some 70+ or - % of police apprehensions DO tend to go to ground, however.
There is a certain kind of fight that doesn't get past the first punch... It's the predatory ambush. Peyton Quinn and Marc "Animal McYoung talk about this at length in their books on self defense.
But in many other contexts this isn't true. Many of the fights one encounters in a law enforcement context are not surprises. The officer(s) move towards an already existing threat and are ready. So they don't get surprised by that first sucker punch. Fights can last quite a long time and officers routinely get hurt.
Officers end up on the ground frequently because they are trying to execute take downs on a resistant subject. When the subject goes down so does the officer. Often this is intentional but other times it has to do with poor body mechanics on the part of the officer who over commits on the take down and loses his own balance when the subject does. Better training would reduce the percentage of encounters that go to the ground in LE involved situations.